Surface acoustic wave filter

ABSTRACT

A SAW resonator disposed with an IDT electrode and reflector electrodes is formed on the surface of a piezoelectric substrate. Three pieces of the SAW resonator form an acoustic coupling by being disposed close to each other, electrode fingers comprising the IDT electrode in the center are all grounded, and IDT electrode fingers are electrically independent on the side to be connected to input-output terminals of the IDT electrodes disposed outside. As a result, three excitation modes having different propagation frequencies are generated which are used to attain a broad band SAW resonator filter. When these three pairs of the IDT electrode are electrically independent, a balanced input-output can be achieved, and excellent out-of-band rejection characteristics can be obtained.

This application is a divisional of U.S. application Ser. No.08/319,790, filed Oct. 7, 1994 now U.S. Pat. No. 5,581,141.

FIELD OF THE INVENTION

This invention relates to a surface acoustic wave filter used for a highfrequency circuit of a radio.

BACKGROUND OF THE INVENTION

A conventional surface acoustic wave (SAW) filter has been usedextensively for communication equipment as a RF- and IF-stage filterwithin the reception and transmission circuits. Along with the recentdevelopment of mobile communication towards becoming digitalized, adigital mobile telephone or a digital cordless telephone has beenintensively developed. In the communication system of these devices, aphase as well as an amplitude convey information, so that it isessential for a filter used for an IF-stage that the filter not only hasexcellent amplitude characteristics but also is flat in group delaydeviation characteristics. Furthermore, this filter is required to haveexcellent selectivity characteristics for distinguishing a signal of aneighboring channel from a desired signal, for which steep out-of-bandrejection characteristics are needed. In addition, along with the recentminiaturization of a set to attain higher mounting density, coupling orinterference between components caused by lack in grounding strength andin screening has become a problem. Therefore, a balanced type circuitfor controlling these influences has been rapidly developed.

Well-known conventional SAW filters which can be used for the IF-stageare a transversal type SAW filter and two types of SAW resonator filterswhich are a longitudinally coupled resonator filter and a transverselycoupled resonator filter. The transversal type SAW filter has excellentgroup delay deviation characteristics. However, the insertion loss andthe size are big, and in addition, the out-of-band rejection is poor. Onthe other hand, the SAW resonator filters have excellent out-of-bandrejection characteristics and are small in insertion loss and size, butthe group delay deviation characteristics are inferior to that of thetransversal type SAW filter. In addition, the longitudinally coupledresonator type is characterized by the large spurious present on theclose and high side of a pass band, while the transversely coupledresonator type is characterized by having extremely narrow-band passcharacteristics. As a conventional IF filter of mobile communication,the transversely coupled resonator type SAW filter was used commonlywhich had a compact size and excellent out-of-band rejectioncharacteristics.

A conventional transversely coupled resonator type SAW filter will beexplained with reference to FIG. 24.

FIG. 24 shows an electrode pattern of a conventional transverselycoupled resonator type SAW filter. Referring to FIG. 24, referencenumeral 241 represents a monocrystal piezoelectric substrate on which anelectrode pattern is formed to generate a surface acoustic wave. 242a isan inter-digital transducer (IDT) electrode which is disposed withreflectors 242b and 242c on both sides to form an energy trapping typeSAW resonator. The same type of SAW resonator is formed by an IDTelectrode 243a and reflectors 243b, 243c. When the above-mentioned tworesonators are closely disposed to each other, an acoustic couplingoccurs between the two resonators, thereby constructing a SAW resonatorfilter of the first stage. A SAW resonator filter of the second stage isconstructed in the same manner as mentioned above by means of IDTelectrodes 244a, 245a and reflectors 244b, 244c, 245b, and 245c. Thesetwo stages of SAW resonator filters are concatenately connectedelectrically through an electrode pattern 246 to comprise a multistageconnected SAW filter.

In case of the SAW filter constructed above, the mode frequencies of thetwo different surface acoustic waves to be excited on the surface of thepiezoelectric substrate are determined through an electrode overlapwidth of the IDT electrode and through a distance between the twoclosely disposed SAW resonators, thereby fixing the pass band width ofthe filter. However, this filter is characterized by its extremelynarrow-band width to be achieved, so the above-noted structure of FIG.24 can realize a fractional band width (band width standardized by thecenter frequency of a filter) of about 0.1% at the very most for thefilter. In addition, since input-output impedance characteristics dependon the size of the above-noted IDT electrode finger overlap width, it isdifficult to achieve optional impedance. Furthermore, the electrodestructure of FIG. 24 can not achieve a balanced input-output due to thefact that the electrode fingers of IDT electrodes 242a, 245a aregrounded on one side.

In order to keep step with the digitalization mentioned above, a flatband in group delay deviation characteristics is required to bebroadened by broadening pass characteristics.

Furthermore, a balanced input-output needs to be attained. In theconventional method, an elongation coil was inserted between stages of afilter and a ground when the band needed to be broadened. A connectionwith surrounding circuits was attained by adding matching circuits atinput-output stages.

This conventional structure, however, had a defect in that the circuititself was large due to an increased number of components, sinceelongation coils or matching circuit elements were connected as theexternal circuits. At the same time, both the differences in andcoupling of these elements affected the filter characteristicsnegatively, and furthermore, the input-output was unbalanced.

SUMMARY OF THE INVENTION

An object of this invention is to solve the above-mentioned problems byproviding a SAW filter which has a compact size, has a broad band andstable characteristics, and is also capable of a balanced typeinput-output.

In order to accomplish these and other advantages, a surface acousticwave filter of the first embodiment of this invention comprises asurface acoustic wave (SAW) resonator disposed with an inter-digitaltransducer (IDT) electrode and reflectors on both sides of the surfaceof a piezoelectric substrate. Here, three pieces of this SAW resonatorform an acoustic coupling by being disposed close to each other inparallel to a propagation direction of a SAW, and the IDT electrodecomprising the SAW resonator positioned in the center is totallygrounded, and the IDT electrodes of the SAW resonators disposed on theoutside are electrically independent.

It is preferable that the IDT electrode comprising the SAW resonatorpositioned in the center is grounded via electrode patterns disposedbetween the IDT electrodes of the SAW resonators disposed on the outsideand electrodes of the reflectors.

Furthermore, it is preferable that a plurality of the filter isconcatenately connected on the surface of the piezoelectric substratethrough an interstage connecting electrode pattern formed thereon.

In addition, it is preferable that a part of the interstage connectingelectrode pattern has an electrode pad formed for bonding.

It is preferable that the interstage connecting electrode pattern isgrounded via a reactive element formed by an electrode pattern on thesurface of the same piezoelectric substrate.

Furthermore, it is preferable that the reactive element is a spiralinductor.

A second embodiment of this invention is a surface acoustic wave filtercomprising a SAW resonator disposed with an IDT electrode and reflectorson both sides of the surface of a piezoelectric substrate. Here, threepieces of the SAW resonator form an acoustic coupling by being disposedclose to each other in parallel to a propagation direction of a SAW, andelectrode patterns for bus bars disposed at two places between theadjacent SAW resonators are divided, and the IDT electrode comprisingthe SAW resonator positioned in the center is totally grounded.

It is preferable that an IDT electrode comprising one of the SAWresonators positioned in the outside is connected to a balanced typeinput terminal, and an IDT electrode comprising the other SAW resonatorpositioned in the outside is connected to a balanced type outputterminal.

Furthermore, it is preferable that the IDT electrode comprising the SAWresonator positioned in the center is grounded via electrode patternsdisposed between the IDT electrodes of the SAW resonators disposed inthe outside and electrodes of the reflectors.

Also, it is preferable that a line width of the electrode patterns forbus bars comprising the IDT electrodes of the SAW resonators positionedin the outside is selected to be larger than a line width of theelectrode patterns for bus bars comprising the IDT electrode of the SAWresonator positioned in the center.

It is preferable that a plurality of the filter is concatenatelyconnected through several interstage connecting electrode patternsformed on the surface of the piezoelectric substrate.

Additionally, it is preferable that a part of the several interstageconnecting electrode patterns has an electrode pad formed for bonding.

Furthermore, it is preferable that the several interstage connectedelectrode patterns are connected to each other via a reactive element.

It is preferable that one of the several interstage connecting electrodepatterns is grounded, and the other is grounded via a reactive element.

Furthermore, it is preferable that the reactive element is a spiralinductor formed by an eletrode pattern disposed on the surface of thesame piezoelectric substrate.

A third embodiment of this invention is a surface acoustic wave filtercomprising a SAW resonator disposed with an IDT electrode and reflectorson both sides on the surface of a piezoelectric substrate. Here, twopieces of the SAW resonator are formed in parallel to the propagationdirection of a SAW, and between the SAW resonators, aperiodic-structured electrode row comprising stripline electrodes havingabout the same length as an IDT electrode overlap width of the SAWresonator. In this instance, the stripline electrodes are disposedparallely at the same electrode period as in the SAW resonator, and theSAW resonators and the periodic electrode row form an acoustic couplingby being disposed close to each other.

It is preferable that each stripline electrode comprising the periodicelectrode row is connected to each other through bus bars disposed onboth edges.

Furthermore, it is preferable that the periodic-structured electrode rowis grounded via electrodes disposed in an aperture between the IDTelectrodes of the SAW resonators positioned in the outside andelectrodes of the reflectors and further via bus bar electrodes.

In addition, it is preferable that an electrode of one SAW resonatorpositioned in the outside is connected to a balanced type inputterminal, and an electrode of the other SAW resonator positioned in theoutside is connected to a balanced type output terminal.

It is preferable that the line width of the electrode patterns for busbars on the adjacent side of the periodic-structured electrode row ofthe IDT electrodes comprising the SAW resonators positioned in theoutside is selected to be larger than the line width of the electrodepatterns for bus bars formed on the periodic-structured electrode row.

It is also preferable that a plurality of the filter is concatenatelyconnected through several interstage connecting electrode patternsformed on the surface of the piezoelectric substrate.

Furthermore, it is preferable that IDT electrodes on the adjacent sideof the periodic-structured electrode row of the SAW resonator areintegrated with bus bar electrodes which connect the periodic-structuredelectrode row, and that the periodic-structured electrode row isgrounded.

Additionally, it is preferable that a plurality of the filter isconcatenately connected through several interstage connecting electrodepatterns formed on the surface of the piezoelectric substrate.

A fourth embodiment of this invention is a surface acoustic wave filtercomprising a SAW resonator disposed with an IDT electrode and reflectorson both sides on the surface of a piezoelectric substrate. Here, twopieces of the SAW resonator form an acoustic coupling by being disposedclose to each other, and electrodes of the SAW resonators comprising thefirst SAW resonator filter are arranged to be opposite in phase, andelectrodes of the SAW resonators comprising the second SAW resonatorfilter are arranged to be equal in phase. In this instance, the firstSAW resonator filter and the second SAW resonator filter are connectedin parallel.

It is preferable that the first and the second SAW resonator filters areconstructed in such a manner that the high band side excitationfrequency of one SAW resonator filter conforms with the low band sideexcitation frequency of the other SAW resonator filter.

A fifth embodiment of this invention is a surface acoustic wave filtercomprising a SAW resonator disposed with an IDT electrode and reflectorson both sides on the surface of a piezoelectric substrate. Here, fourpieces of the SAW resonator form an acoustic coupling by being disposedclose to each other, and electrodes of the SAW resonators comprising thefirst and the second SAW resonator filters are arranged to be oppositein phase, and electrodes of the SAW resonators comprising the third andthe fourth SAW resonator filters are arranged to be equal in phase,wherein the first SAW resonator filter and the third SAW resonatorfilter are parallel-connected and the second and the fourth SAWresonator filters are parallel-connected. In this instance, the firstand the third SAW resonator filters and the second and the fourth SAWresonator filters are concatenately connected through electrode patternsformed between the filters on the surface of the piezoelectricsubstrate.

It is also preferable that the first and the third SAW resonator filtersare constructed in such a manner that the high band side excitationfrequency of one SAW resonator filter conforms with the low band sideexcitation frequency of the other SAW resonator filter, and the secondand the fourth SAW resonator filters are constructed in such a mannerthat the high band side excitation frequency of one SAW resonator filterconforms with the low band side excitation frequency.

In addition, it is preferable that the first SAW resonator filter andthe second SAW resonator filter are positioned next to each other inparallel to the propagation direction of the surface acoustic wave, andthe third SAW resonator filter and the fourth SAW resonator filter arepositioned next to each other in parallel to the propagation directionof the surface acoustic wave.

A sixth embodiment of this invention is a surface acoustic wave filtercomprising a SAW resonator disposed with an IDT electrode and reflectorson both sides on the surface of a piezoelectric substrate which forms anacoustic coupling by being disposed close to each other. Here, electrodepatterns for bus bars are divided at the central part of an adjacent IDTelectrode in the SAW resonator filter, and a plurality of the SAWresonator disposed has electrically independent bus bars.

It is preferable that two pieces of SAW filter are formed on the samepiezoelectric substrate, and an electrode of the SAW resonatorcomprising the first SAW resonator filter is arranged to be opposite inphase, and an electrode of the SAW resonator comprising the second SAWresonator filter is arranged to be equal in phase. Here, the first SAWresonator filter and the second SAW resonator filter areparallel-connected.

Furthermore, it is preferable that the first and the second SAWresonator filters are constructed in such a manner that the high bandside excitation frequency of one SAW resonator filter conforms with thelow band side excitation frequency of the other SAW resonator filter.

A seventh embodiment of this invention is a surface acoustic wave filtercomprising a SAW resonator disposed with an IDT electrode and reflectorson both sides on the surface of a piezoelectric substrate which forms anacoustic coupling by being disposed close to each other. Here, electrodepatterns for bus bars are divided at the central part of the SAWresonator filter, and four pieces of the SAW resonator filters wereformed such that electrodes of the SAW resonators comprising the firstand the second SAW resonator filter are arranged to be opposite inphase, and electrodes of the SAW resonators comprising the third and thefourth SAW resonator filter are arranged to be equal in phase. The firstSAW resonator filter and the third SAW resonator filters areparallel-connected and the second and the fourth SAW resonator filtersare parallel-connected. In this instance, the first and the third SAWresonator filters are concatenately connected to the second and thefourth SAW resonator filters through electrode patterns formed betweenthe filters on the surface of the piezoelectric substrate.

It is preferable that the first and the third SAW resonator filters areconstructed in such a manner that the high band side excitationfrequency of one SAW resonator filter conforms with the low band sideexcitation frequency of the other SAW resonator filter. Then, the secondand the fourth SAW resonator filters are constructed in such a mannerthat the high band side excitation frequency of one SAW resonator filterconforms with the low band side excitation frequency of the other SAWresonator filter.

An eighth embodiment of this invention is a surface acoustic wave filtercomprising a reactive element formed by using a part of electrodepatterns of the SAW filter.

It is also preferable that the surface acoustic wave filter comprises areactive element disposed with an IDT electrode and reflector electrodesand is formed by using the reflector electrodes.

In addition, it is preferable that the reactive element is an inductorformed by connecting parallel-positioned stripline electrodes comprisingthe reflector electrodes in a zigzag pattern.

Furthermore, it is preferable that the reactive element is an inductorformed by bundling and connecting a plurality of parallel-positionedstripline electrodes comprising the reflector electrodes in a zigzagpattern.

It is preferable that the reactive element is a capacitor formed byconnecting parallel-positioned stripline electrodes comprising thereflector electrodes in an inter-digital form.

It is also preferable that the reactive element is used to form aninput-output matching circuit.

Additionally, it is preferable that the reactive element is used to forman interstage matching circuit.

It is preferable that a plurality of SAW resonator comprising an IDTelectrode and reflectors on both sides forms an acoustic coupling bybeing disposed close to each other.

Furthermore, it is preferable that the reactive element is formed byusing a reflector electrode.

A ninth embodiment of this invention is a surface acoustic wave filtercomprising a SAW resonator disposed with an IDT electrode and reflectorson both sides. Here, a plurality of the SAW resonator forming anacoustic coupling by being disposed close to each other are formed onthe surface of the same piezoelectric substrate, and the SAW resonatorfilters are concatenately connected, and an input-output matchingcircuit is formed by using a reactive element formed by an electrodepattern disposed on the surface of the piezoelectric substrate.

It is preferable that the reactive element is formed by using reflectorelectrodes.

A tenth embodiment of this invention is a surface acoustic wave filtercomprising a SAW resonator disposed with an IDT electrode and reflectorson both sides. Here, a plurality of the SAW resonator forming anacoustic coupling by being disposed close to each other are formed onthe surface of the same piezoelectric substrate, and the SAW resonatorfilters are concatenately connected, and the connecting points aregrounded via a reactive element formed by an electrode pattern disposedon the surface of the same piezoelectric substrate.

It is preferable that the reactive element is a spiral inductor.

Furthermore, it is preferable that the spiral inductor is formed byusing an aperture between the plurality of SAW filters.

It is also preferable that a short-circuit electrode pattern for ashort-circuit connection between the winding electrode patterns adjacentto the spiral inductor is disposed at least at one place.

According to the first embodiment of this invention, three pieces of aSAW resonator disposed with an inter-digital transducer (IDT) electrodeand reflectors on both sides on the surface of a piezoelectric substrateform an acoustic coupling by being disposed close to each other inparallel to the propagation direction of a SAW, and the IDT electrodecomprising the SAW resonator positioned in the center is totallygrounded, and furthermore, the IDT electrodes of the SAW resonatorsdisposed in the outside are electrically independent. Since thepotential of the SAW resonator positioned in the center can bedistributed freely, and potential between the SAW resonators disposed inthe outside is not canceled, a strong vibration strength can be obtainedas well with respect to the second-order mode. As a result, a SAW filterusing three excitation modes can be constructed, and this filter canachieve broader pass band characteristics than the conventional SAWfilter with two excitation modes without using an external elongationcoil etc. Additionally, the SAW filter using three excitation modes isattained here, so that this filter has steeper out-of-band rejectionfactor than that of the conventional SAW filter in the second-ordermode. Thus, better selectivity characteristics can be obtained as well.

Furthermore, according to the first embodiment, it is preferable thatthe IDT electrode comprising the SAW resonator positioned in the centeris grounded via electrode patterns disposed in an aperture between theIDT electrodes of the SAW resonators disposed in the outside andelectrodes of the reflectors. Thus, the distance between the IDTelectrode and the reflector comprising the SAW filter can be determinedwith greater freedom. Therefore, the out-of-band interference can besuppressed by suitably selecting the distance between the IDT electrodeand the reflector. As a result, better out-of-band characteristics canbe obtained in this way.

In addition, it is preferable that a plurality of the filter isconcatenately connected through an interstage connecting electrodepattern formed on the surface of the piezoelectric substrate.Accordingly, the out-of-band rejection characteristics can be improvedconsiderably, so even better filter characteristics can be obtained.Here, when an electrode pad for bonding is formed at a part of theinterstage connecting electrode pattern, the connection between theinterstage connecting electrode pattern and an external circuit can besimplified for obtaining good pass characteristics by connecting amatching element such as an inductor to the interstage connectingpattern. In this instance, it is also preferable that the interstageconnecting electrode pattern is grounded via a reactive element formedby an electrode pattern disposed on the same surface of thepiezoelectric substrate. Thus, an external circuit is no longer neededso that a compact-sized filter circuit can be attained. Furthermore,when this reactive element is a spiral inductor, the reactive elementcan be miniatuarized.

According to the second embodiment of this invention, a surface acousticwave filter comprises a SAW resonator disposed with an IDT electrode andreflectors on both sides on the surface of a piezoelectric substrate,and three pieces of the SAW resonator form an acoustic coupling by beingdisposed close to each other in parallel to the propagation direction ofa SAW, and electrode patterns for bus bars disposed at two placesbetween the adjacent SAW resonators are divided, and the IDT electrodecomprising the SAW resonator positioned in the center is totallygrounded. Thus, the bus bars in the central part of the IDT electrodecan become electrically independent, and all the IDT electrodes of theSAW filters disposed in the outside can be wired independently.Therefore, among the IDT electrodes comprising the SAW filter, only theIDT electrode of the SAW resonator positioned in the center is grounded,so that input-output terminals can become electrically independent atthis part. As a result, input-output characteristics of the filter arenot directly affected by how the IDT electrodes are grounded, andfurthermore, since direct components of signals between the input-outputterminals decrease considerably, out-of-band rejection characteristicsof the filter can be improved even more.

Also, it is preferable that an IDT electrode comprising one of the SAWresonators positioned in the outside is connected to a balanced typeinput terminal, and an IDT electrode comprising the other SAW resonatorpositioned in the outside is connected to a balanced type outputterminal. Accordingly, a balanced type element such as an integratedcircuit (IC) comprising differential if-amplifiers, for example, can beelectrically connected upstream and downstream to the filter withoutusing an outside circuit of balun or the like. In this way, noisecharacteristics of the whole circuit can be improved. In this instance,when the line width of the electrode patterns for bus bars comprisingthe IDT electrodes of the SAW resonators positioned in the outside isselected to be larger than a line width of the electrode patterns forbus bars comprising the IDT electrode of the SAW resonator positioned inthe center, insertion loss of the filter can be improved. The reason isas follows. G, the distance between the comb-formed electrodescomprising the IDT electrodes of the adjacent SAW resonators, controlsthe coupling degree of the two SAW resonators. The smaller this distanceis, the stronger the coupling degree between the SAW resonators becomes,which is preferable for attaining a broader band. If G becomes toosmall, however, widths W1 and W2 of the bus bar electrodes disposed inthis part become smaller, so that the electrical resistance loss of theIDT electrodes in this part affects the insertion loss of the filterconsiderably. Here, the bus bar electrode disposed in the outside (linewidth W1) is directly connected to one of the balanced input-outputterminals, but the bus bar electrode disposed in the central part (linewidth W2) is used for grounding the IDT electrode comprising the SAWresonator in the center. Thus, they are not electrically connected tothe input-output terminals. In other words, line width W2 of the bus barelectrode positioned in the center does not affect the insertion loss ofthe filter at all. Therefore, when line width W2 of the bus barelectrode positioned in the center is selected to be smaller, line widthW1 of the bus bar electrode disposed in the outside can be selected tobe larger for compensation. As a result, the electrical resistance lossinto the input-output terminals can be reduced without changing thedistance G between the comb-formed electrodes comprising the IDTelectrodes of the adjacent SAW resonators. In this way, the insertionloss of the filter can be improved.

Furthermore, according to the second embodiment, it is preferable thatthe IDT electrode comprising the SAW resonator positioned in the centeris grounded via electrode patterns disposed between the IDT electrodesof the SAW resonators disposed in the outside and the reflectorelectrodes, the distance between the IDT electrode and the reflectorscomprising the SAW filter can be determined with greater freedom. Thus,the out-of-band interference can be suppressed by suitably selecting thedistance between the IDT electrode and the reflectors. As a result,better out-of-band characteristics can be obtained.

Also, it is preferable that a plurality of the filter is concatenatelyconnected through several interstage connecting electrode patternsformed on the surface of the piezoelectric substrate. Accordingly,out-of-band rejection characteristics can be improved considerably sothat even better filter characteristics can be obtained. Here, it ispreferable that a part of the several interstage connecting electrodepatterns is formed with an electrode pad for bonding. When good passcharacteristics are obtained by connecting a matching element such as aninductor to the interstage connecting electrode patterns, the connectionbetween the interstage connecting electrode patterns and an externalcircuit is simplified. In this instance, it is preferable that theseveral interstage connecting electrode patterns are connected to eachother via a reactive element for obtaining good pass characteristics.Also, it is preferable that one of the several interstage connectingelectrode patterns is grounded, and another is grounded via a reactiveelement for obtaining good pass characteristics. Furthermore, it ispreferable that the reactive element is a spiral inductor formed by anelectrode pattern disposed on the surface of the same piezoelectricsubstrate for obtaining a reactive element having a compact size.

According to the third embodiment of this invention, a surface acousticwave filter comprises a SAW resonator disposed with an IDT electrode andreflectors on both sides of the surface of a piezoelectric substrate,and two parts of the SAW resonator are formed in parallel to apropagation direction of a surface acoustic wave, and between the SAWresonators, a periodic-structured electrode row is formed comprisingstripline electrodes having about the same length as an IDT electrodeoverlap width of the SAW resonator which are parallel-disposed at thesame electrode period as of the SAW resonator, and the SAW resonatorsand the periodic-structured electrode row form an acoustic coupling bybeing disposed close to each other. Therefore, even if the electrodestructure of the SAW resonator positioned in the center is changed fromthe IDT electrode to the periodic-structured stripline electrode row,when the electrode cycle is the same, the surface acoustic wave can bepropagated as in the first embodiment. As a result, a broader band ofthe SAW filter can be attained.

Furthermore, it is preferable in this embodiment that each striplineelectrode comprising the periodic-structured electrode row is connectedto each other through bus bars disposed on both edges of the electroderow. Accordingly, the periodic-structured electrode row can bestructured as the reflector. In addition, it is preferable that the IDTelectrode on the side adjacent to the periodic-structured electrode rowof the SAW resonator and the bus bar electrode connecting theperiodic-structured electrode row are combined into one, and that theperiodic-structured electrode row is grounded. As a result, thisstructure can attain an unbalanced input-output as in the above-notedfirst embodiment, and also, a broader band of the SAW filter can beattained.

When a plurality of this filter is concatenately connected viainterstage connecting electrode patterns formed on the surface of thepiezoelectric substrate, the out-of-band rejection characteristics canbe improved considerably so that good filter characteristics can beobtained. Here, it is preferable that the periodic-structured electroderow is grounded via electrodes disposed in an aperture between the IDTelectrodes of the SAW resonators positioned in the outside and via busbar electrodes. Accordingly, the periodic-structured electrodes becomeelectrically independent from the input-output terminals. As a result,input-output characteristics of the filter are not affected by how theperiodic-structured electrodes are grounded, and furthermore, directcomponents of signals between the input-output terminals decreaseconsiderably. Thus, the out-of-band rejection characteristics of thefilter can be improved even more, as in the above-mentioned secondembodiment. Also, since the distance between the IDT electrode as wellas the periodic-structured electrode and the reflectors comprising theSAW resonator can be determined with greater freedom, the out-of-bandinterference can be suppressed by suitably selecting the distancebetween the IDT electrode as well as the periodic-structured electrodeand the reflectors. As a result, better out-of-band characteristics canbe obtained in this way.

According to this embodiment, it is preferable that an electrode of oneSAW resonator positioned in the outside is connected to a balanced typeinput terminal and an electrode of the other SAW resonator positioned inthe outside is connected to a balanced type output terminal. As aresult, a balanced type element such as an IC can be connected upstreamand downstream to the filter without using an external circuit of balunor the like so that noise characteristics of the whole circuit areimproved as well. Furthermore, when a line width of the electrodepatterns for bus bars formed on the adjacent side of theperiodic-structured electrode of the IDT electrodes comprising the SAWresonators positioned in the outside is selected to be larger than aline width of the electrode patterns for bus bars formed on theperiodic-structured electrode row, insertion loss of the filter can beimproved even more, as in the above-noted second embodiment. This is dueto the fact that when compared with the second embodiment, a portion ofthe part where no electrode is present decreases in G which is thedistance between the comb-formed electrodes comprising the IDTelectrodes of the SAW resonators disposed in the outside and theperiodic-structured electrode disposed in the center. Instead, the linewidth W1 of the electrode pattern for the bus bar on the side adjacentto the periodic-structured electrode of the IDT electrode comprising theSAW resonator disposed in the outside can be enlarged.

Additionally, it is preferable in this embodiment that a plurality ofthe filter is concatenately connected through several interstageconnecting electrode patterns formed on the surface of the piezoelectricsubstrate. As a result, out-of-band rejection characteristics can beimproved considerably, and better filter characteristics can beobtained.

According to the fourth embodiment of this invention, two portions ofthe SAW resonator disposed with an IDT electrode and reflectors on bothsides forming an acoustic coupling by being disposed close to each otherare formed on the surface of a piezoelectric substrate, and electrodesof the SAW resonators comprising the first SAW resonator filter arearranged to be opposite in phase, and electrodes of the SAW resonatorscomprising the second SAW resonator filter are arranged to be equal inphase, and the first SAW resonator filter is connected parallel to thesecond SAW resonator filter. As a result, the band width can bebroadened without deteriorating the pass characteristics of the band.This is because a single SAW resonator filter has two excitationfrequencies of either f₁ and f₂ (f₁ <f₂) or f³ and f₄ (f³ <f₄), and thephase relationship between these two frequencies is opposite. Therefore,according to this embodiment, the electrodes of the SAW resonatorcomprising the first SAW resonator filter are arranged to be opposite inphase, and the electrodes of the SAW resonator comprising the second SAWresonator filter are arranged to be equal in phase, and the first SAWresonator filter is connected parallel to the second SAW resonatorfilter. Thus, the phase relationship between f₁ and f₂ as well as thephase relationship between f₃ and f₄ can be reversed. In other words, f₂and f₃ become equal in phase. As a result, by conforming the excitationfrequencies of f₂ and f₃, a band width can be broadened withoutdeteriorating the pass characteristics of the band.

According to the fifth embodiment of this invention, four parts of a SAWresonator disposed with an IDT electrode and reflectors on both sidesforming an acoustic coupling by being disposed close to each other areformed on the surface of a piezoelectric substrate, and electrodes ofthe SAW resonators comprising the first and the second SAW resonatorfilters are arranged to be opposite in phase, and electrodes of the SAWresonators comprising the third and the fourth SAW resonator filters arearranged to be equal in phase, and the first SAW resonator filter isconnected parallel to the third SAW resonator filter and the second SAWresonator filter is connected parallel to the fourth SAW resonatorfilter, and the first and the third SAW resonator filters and the secondand the fourth SAW resonator filters are concatenately connected throughelectrode patterns formed between the filters on the surface of thepiezoelectric substrate. As a result, two pairs of theparallel-connected SAW resonators are operated respectively in the samemanner as in the first embodiment. By concatenately connecting theseresonators, the out-of-band rejection can be improved even more.

Furthermore, it is preferable in this embodiment that the first and thethird SAW resonator filters are constructed in such a manner that thehigh band side excitation frequency of one SAW resonator filter conformswith the low band side excitation frequency of the other SAW resonatorfilter, and that the second and the fourth SAW resonator filters areconstructed in such a manner that the high band side excitationfrequency of one SAW resonator filter conforms with the low band sideexcitation frequency of the other SAW resonator filter. As a result,band width can be broadened without deteriorating the passcharacteristics of the band, as in the fourth embodiment.

In addition, it is preferable that the first SAW resonator filter andthe second SAW resonator filter are positioned next to each other inparallel to the propagation direction of the surface acoustic wave, andthe third SAW resonator filter and the fourth SAW resonator filter arepositioned next to each other in parallel to a propagation direction ofthe surface acoustic wave. Accordingly, undesired stray capacitance canbe eliminated between the concatenately connected multistage connectedSAW filters, and therefore, an interstage matching circuit is no longernecessary. As a result, the circuit can be compact-sized, while stablecharacteristics of the filter can be achieved.

According to the sixth embodiment of the invention, a SAW resonatordisposed with an IDT electrode and reflectors on both sides on thesurface of a piezoelectric substrate forms acoustic coupling by beingdisposed close to each other, and electrode patterns for bus bars aredivided at the central part of an IDT electrode adjacent to the SAWresonator filter, and a plurality of the SAW resonator disposed closelyhas electrically independent bus bars. Thus, ground electrodes of theinput IDT electrode and the output IDT electrode can be taken outindependently, and a balanced input-output of the SAW filter can beachieved. As a result, since the IDT electrodes of the SAW filter are nolonger needed to be grounded, the input-output characteristics of thefilter are not directly affected by how the SAW electrodes are grounded.Furthermore, since direct components of signals between the input-outputterminals decrease considerably, the out-of-band rejectioncharacteristics of the filter can be improved. Also, a balanced typeelement such as IC can be connected upstream and downstream to thefilter without using an external circuit of balun etc. so that noisecharacteristics of the whole circuit are improved as well.

Furthermore, it is preferable that two pieces of the SAW filter areformed on the same piezoelectric substrate, and electrodes of the SAWresonator comprising the first SAW resonator filter are arranged to beopposite in phase, and electrodes of the SAW resonator comprising thesecond SAW resonator filter are arranged to be equal in phase, and thefirst SAW resonator filter and the second SAW resonator filter areparallel-connected. Thus, the band width can be broadened withoutdeteriorating the pass characteristics of the band as in the fourthembodiment.

According to the seventh embodiment of this invention, a surfaceacoustic wave filter comprises a SAW resonator disposed with an IDTelectrode and reflectors on both sides on the surface of a piezoelectricsubstrate forming acoustic coupling by being disposed close to eachother, and electrode patterns for bus bars are divided at the centralpart of the SAW resonator filter, and four pieces of the SAW resonatorfilters are formed in such a manner that electrodes of the SAWresonators comprising the first and the second SAW resonator filter arearranged to be opposite in phase, and electrodes of the SAW resonatorscomprising the third and the fourth SAW resonator filter are arranged tobe equal in phase, and the first SAW resonator filter isparallel-connected to the third SAW resonator filter and the second SAWresonator filter is connected parallel to the fourth SAW resonatorfilter, and the first and the third SAW resonator filters areconcatenately connected to the second and the fourth SAW resonatorfilters through electrode patterns formed between the filters on thesurface of the piezoelectric substrate. Thus, two pairs ofparallel-connected SAW resonator filters are operated in the same manneras in the above-noted sixth embodiment. By concatenately connectingthese filter, the out-of-band rejection can be improved even more.

According to the eighth embodiment of this invention, a reactive elementis formed by using a part of electrode patterns comprising the SAWfilter. In this way, it is not necessary to add a separate electrodearea so that the above-noted circuit structure can be achieved withabout the same piezoelectric substrate area as in a conventional SAWfilter.

It is also preferable that the surface acoustic wave filter disposedwith an IDT electrode and reflector electrodes comprises a reactiveelement formed by using the reflector electrodes. In this way,differences in element values when compared with using an exteriorcircuit element can be reduced, so circuit characteristics of the SAWfilter can be stabilized. Here, it is preferable that the reactiveelement is an inductor formed by connecting parallel-positionedstripline electrodes comprising the reflector electrodes in a zigzagpattern for attaining a compact-sized reactive element. Furthermore, itis preferable that the reactive element is an inductor formed bybundling and connecting a plurality of parallel-positioned striplineelectrodes comprising the reflector electrodes in a zigzag pattern. As aresult, the resistance component of the inductor can be reduced, sodeterioration of filter characteristics can be prevented. It is alsopreferable that the reactive element is a capacitor formed by connectingparallel-positioned stripline electrodes comprising the reflectorelectrodes in an inter-digital form. In this way, the reactive value canbe finely adjusted by trimming the electrode patterns.

It is also preferable that the reactive element is used to form aninput-output matching circuit or to form an interstage matching circuit.Accordingly, the reactive electrode patterns serve as matching circuitsfor the SAW filter, and as a result, it is no longer necessary toinstall an exterior matching circuit. Thus, a number of components canbe reduced to attain a compact circuit as a whole.

Additionally, it is preferable that a plurality of the SAW resonatorcomprising an IDT electrode and reflectors on both sides forms anacoustic coupling by being disposed close to each other. In this way, anenergy trapping type SAW resonator filter can be attained with about thesame piezoelectric substrate area as in the conventional filter.Furthermore, it is preferable that the reactive element is formed byusing a reflector electrode for reducing differences in element valueswhen compared with the case using an exterior circuit element. Thus, thecircuit characteristics of the SAW filter can be stabilized.

According to the ninth embodiment, a plurality of a SAW resonatordisposed with an IDT electrode and reflectors on both sides forming anacoustic coupling by being disposed close to each other is formed on thesurface of the same piezoelectric substrate, and the SAW resonatorfilters are concatenately connected, and input-output matching circuitsare formed by using a reactive element formed by an electrode patterndisposed on the surface of the piezoelectric substrate. Thus,differences in element values can be reduced when compared with the caseusing an exterior circuit element, so that circuit characteristics ofthe multistage connected SAW filter can be stabilized.

According to the tenth embodiment of this invention, a SAW resonatorfilter comprises a plurality of SAW resonators disposed with an IDTelectrode and reflectors on both sides forming an acoustic coupling bybeing disposed close to each other, and a plurality of the SAW filter isformed on the surface of the same piezoelectric substrate, and theabove-noted SAW resonator filters are concatenately connected, and theconnecting points are grounded via reactive elements formed by electrodepatterns disposed on the surface of the same piezoelectric substrate. Inthis way, the reactive elements formed by the electrode patterns on thesurface of the piezoelectric substrate serve as interstage matchingelements for the filter. In this way, an interstage unadjustment of theSAW filter can be achieved without increasing the electrode area on thesurface of the piezoelectric substrate. It is no longer necessary toconnect an adjustment circuit such as an elongation coil at theinterstage of, e.g., a broadband type transversely coupled resonatortype SAW filter.

According to the tenth embodiment, it is preferable that the reactiveelement is a spiral inductor for attaining a small-sized reactiveelement. It is also preferable that the spiral inductor is formed byusing an aperture between the plurality of SAW filters. In this way, itis unnecessary to enlarge the piezoelectric substrate more than theconventional one, so the circuit can be miniatuarized. Furthermore, whena short-circuit electrode pattern is formed at least at one part for ashort-circuit connection of winding electrode patterns adjacent to thespiral inductor, the reactive value can be finely controlled by trimmingthe short-circuit electrode pattern by means of a laser or the like. Asa result, the filter characteristics can be finely controlled after theSAW filter substrate is mounted to the package.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a surface acoustic wave filter of a firstembodiment of this invention.

FIG. 2(a) is the same schematic view of a surface acoustic wave filteras in FIG. 1. FIG. 2(b) is a diagram explaining the distribution ofvibration mode patterns during an operation of a surface acoustic wavefilter of a first embodiment of this invention.

FIGS. 3(a) and (b) are graphs showing pass band characteristics duringoperations of a surface acoustic wave filter of a first embodiment ofthis invention.

FIG. 4 is a schematic view of a multistage connected surface acousticwave filter of a first embodiment of this invention.

FIG. 5 is a schematic view of a surface acoustic wave filter of a secondembodiment of this invention.

FIG. 6 is an enlarged view of a part where IDT electrodes of a surfaceacoustic wave filter are disposed closely to each other of a secondembodiment of this invention.

FIG. 7 is a schematic view of a multistage connected surface acousticwave filter of a second embodiment of this invention.

FIG. 8 is a schematic view of a surface acoustic wave filter of a thirdembodiment of this invention.

FIG. 9 is an enlarged view of a part where IDT electrodes of a surfaceacoustic wave filter are closely disposed to each other of a thirdembodiment of this invention.

FIG. 10 is a schematic view of a part where IDT electrodes of a surfaceacoustic wave filter are arranged to be opposite in phase of a thirdembodiment of this invention.

FIG. 11 is a schematic view of a surface acoustic wave filter in anunbalanced input-output condition of a third embodiment of thisinvention.

FIG. 12 is a schematic view of a surface acoustic wave filter of afourth embodiment of this invention.

FIGS. 13(a) and (b) are graphs showing pass band characteristics duringoperations of a surface acoustic wave filter of a fourth embodiment ofthis invention.

FIG. 14 is a schematic view of a surface acoustic wave filter of a fifthembodiment of this invention.

FIG. 15 is a schematic view of a surface acoustic wave filter of a sixthembodiment of this invention.

FIG. 16 is a view of a surface acoustic wave filter mounted to a facemounting package of a sixth embodiment of this invention.

FIG. 17 is a schematic view of a surface acoustic wave filter of aseventh embodiment of this invention.

FIG. 18 is an equivalent circuit view of a surface acoustic wave filterof a seventh embodiment of this invention.

FIG. 19 is a schematic view of a surface acoustic wave filter of aneighth embodiment of this invention.

FIG. 20 is a schematic view of a surface acoustic wave filter of a ninthembodiment of this invention.

FIG. 21 is an equivalent circuit view of a surface acoustic wave filterof a tenth embodiment of this invention.

FIG. 22 is a schematic view of a surface acoustic wave filter of a tenthembodiment of this invention.

FIGS. 23(a)-(c) are schematic views of reactive electrode patterns of aseventh, eighth, ninth, and tenth embodiment of this invention.

FIG. 24 is a schematic view of a conventional surface acoustic wavefilter.

DETAIL DESCRIPTION OF THIS INVENTION

The invention will be explained in detail with reference to the attachedfigures and the following examples. The examples are illustrative andshould not be construed as limiting the invention in any way.

Example 1

FIG. 1 shows a schematic view of a surface acoustic wave filter of afirst embodiment. Referring to FIG. 1, reference numeral 11 represents amonocrystal piezoelectric substrate. By forming periodic-structuredstripline electrode patterns on the surface of piezoelectric substrate11, a surface acoustic wave can be generated. On the surface ofpiezoelectric substrate 11, there is a first energy trapping type SAWresonator formed by an IDT electrode 12a and reflectors 12b, 12c. Alsoon the same piezoelectric substrate 11, there are formed a second SAWresonator comprising an IDT electrode 13a and reflectors 13b, 13ctogether with a third SAW resonator comprising an IDT electrode 14a andreflectors 14b, 14c in the same manner. The three SAW resonatorsmentioned above are closely disposed to each other, and the adjacent IDTelectrodes and the adjacent electrodes of reflectors are connectedthrough common bus bars. In addition, an outside electrode finger of IDTelectrode 12a is connected to an input terminal, while an outsideelectrode finger of IDT electrode 14a is connected to an outputterminal. Furthermore, one electrode finger of IDT electrode 13a isgrounded together with an inside electrode finger of IDT electrode 12avia an electrode pattern disposed between the common bus bar and anaperture of IDT electrode 12a and reflector 12b. The other electrodefinger of 13a is grounded together with an inside electrode finger ofIDT electrode 14a via an electrode pattern disposed between the commonbus bar and an aperture of IDT electrode 14a and reflector 14c.

Next, the surface acoustic wave filter constructed in the above mannerwill be explained with regard to its operation.

FIG. 2(a) is a diagram showing vibration mode patterns of a surfaceacoustic wave filter of this embodiment. The same reference numerals aregiven to the corresponding parts of FIG. 1. In FIG. 2(a), 21 is anelectrode structure of the SAW filter shown in FIG. 1, and an acousticcoupling occurs between SAW resonators 12, 13, and 14 when they aredisposed closely to each other. At this time, modes in first-order,second-order, and third-order with potential distributions shown as 22are generated as shown in FIG. 2(b). In this instance, the second-ordermode falls on a part where a node of the mode distribution crosses withthe IDT electrode of SAW resonator 13 disposed in the middle, and apolarity of the potential distribution changes at its upper and lowersides. Therefore, when the electrode pattern is only constructed as 21,vibration strength in the second-order mode becomes considerably weakerthan the first-order and third-order modes. Pass band characteristics ofthis filter at direct-coupled 50Ω show a depression in the center asshown in FIG. 3(a). When the frequency difference between thefirst-order mode and the third-order mode is designed to be broad inband which exceeds 0.1% in a normalized state through a centerfrequency, the band will not be flat within even with a matchingcircuit, so that filter characteristics are not satisfactory. Therefore,in order to obtain good filter characteristics in the electrodestructure shown in FIG. 1, the second-order mode needs to be vibratedstrongly for using it for pass characteristics. For this purpose, it isnecessary that the potential of SAW resonator 13 disposed in the centercan be distributed freely, and that potential between SAW resonators 12and 14 in the outside is not canceled. In this embodiment, IDTelectrodes of SAW resonator 13 are both grounded, and IDT electrodefingers 23 and 24 shown in FIG. 2(a) are respectively connected to aninput terminal and an output terminal and are electrically independent.Accordingly, this embodiment satisfies the above-mentioned requirements.Here, the pass band characteristics at direct-coupled 50Ω are shown inFIG. 3(b) which indicates that a strong vibration strength with regardto the second-order mode can be obtained. As a result, a SAW filterusing three excitation modes can be constructed, and this filter canachieve broader pass band characteristics than the conventional SAWfilter using two excitation modes.

Furthermore, the distance between the IDT electrode and the reflector ofa SAW filter influences the strength in out-of-band interference so thatthe interference can be suppressed by selecting the size suitably. Inthis embodiment, by disposing the electrode pattern for grounding theIDT electrode of the central SAW resonator in this part, the distancecan be determined with greater freedom, the out-of-band interference issuppressed, and better out-of-band characteristics can be obtained.

Additionally, the SAW filter of this embodiment makes use of threeexcitation modes, so that the filter becomes third-order. As a result,this filter has steeper out-of-band shape factor than that of theconventional SAW filter of second-order, thereby obtaining betterselectivity characteristics.

According to the above-mentioned embodiment, the band of the SAW filtercan be broadened without using an external elongation coil etc. byclosely disposing three pieces of SAW resonators and grounding all ofthe IDT electrodes comprising the SAW resonator in the center.

Furthermore, this embodiment referred only to a SAW filter having a onestage structure, but as shown in FIG. 4, a plurality of SAW filters 42and 43 can be concatenately connected on the surface of the samepiezoelectric substrate 41 to form a multistage connected SAW filter. Inthis case, even though the insertion loss increases slightly, theout-of-band rejection characteristics can be improved considerably, soeven better filter characteristics can be obtained.

Also, input-output impedance of a SAW filter is controlled by a numberof IDT pairs of a SAW resonator and can not be determined optionally.Therefore, when the filter is simply concatenately connected, passcharacteristics are not always satisfactory due to a mismatchingconnection. In this case, an electrode pattern 44 which is interstageconcatenately connected should be connected to a matching element suchas an inductance. Here, connection with an external circuit issimplified when an electrode pad 45 for bonding is disposed to electrodepattern 44. Or, by forming a reactive element such as a spiral inductoron the same piezoelectric substrate and by connecting one end toelectrode pattern 44 while grounding the other end, an external circuitis no longer needed so that a compact-sized filter circuit can beattained.

This embodiment referred to a description of three SAW resonatorsdisposed closely to each other. Theoretically, this number can beincreased to four or more for constructing a filter using a morehigh-order mode. However, an increased number of resonator complicatesthe filter design, and element sensitivity of the matching circuitrises. Therefore, filter characteristics are no longer satisfactory.Thus, it is preferable to select three pieces of SAW resonators whichare closely disposed.

EXAMPLE 2

FIG. 5 is a schematic view of a surface acoustic wave filter of a secondembodiment of this invention. Referring to FIG. 5, a first SAW resonatoris formed by disposing an IDT electrode 52a and reflectors 52b, 52c onthe surface of a monocrystal piezoelectric substrate 51. Also, on thesurface of piezoelectric substrate 51, a second SAW resonator is formedcomprising an IDT electrode 53a and reflectors 53b, 53c as well as athird SAW resonator comprising an IDT electrode 54a and reflectors 54b,54c. The three SAW resonators mentioned above are disposed close to eachother. Furthermore, one electrode finger of IDT electrode 53a isgrounded between IDT electrode 52a and reflector 52b via an electrodepattern disposed in an aperture, while the other electrode finger isgrounded between IDT electrode 54a and reflector 54c via an electrodepattern disposed in an aperture. The above-mentioned structure isidentical with that of the first embodiment shown in FIG. 1.

This figure differs from FIG. 1 in that adjacent bus bar electrodes ofIDT electrodes comprising the SAW resonators are electricallyindependent. In addition, the electrode finger of IDT electrode 52a isconnected to a balanced type input terminal, whereas the electrodefinger of IDT electrode 54a is connected to a balanced type outputterminal.

The operation of the surface acoustic wave filter constructed above isbasically the same with that of the first embodiment shown in FIG. 2.Thus, a broader band of the SAW filter can be achieved, and out-of-bandinterference can be suppressed. Not only that, since the filter of thisembodiment is constructed in such a manner that the bus bars in thecentral part of the IDT electrodes are electrically independent, all ofthe IDT electrodes in SAW resonator 52 and the IDT electrodes in SAWresonator 54 can be wired independently. As a result, by configuringterminals as shown in FIG. 5, a balanced input-output of the SAW filterwill be possible.

According to this embodiment, among the IDT electrodes comprising theSAW filter only the IDT electrode of SAW resonator 53a in the center isgrounded, and at this part input-output terminals are electricallyindependent. Therefore, input-output characteristics of the filter arenot directly affected by how the IDT electrodes are grounded, andfurthermore, since direct components of signals between the input-outputterminals decrease considerably, out-of-band rejection characteristicsof the filter are improved even more. Also, a balanced type element suchas an IC can be connected upstream and downstream to the filter withoutusing an outside circuit of balun or the like. Thus, noisecharacteristics of the whole circuit are improved as well.

FIG. 6 is an enlarged view of a part where IDT electrodes 52a and 53aare closely disposed in FIG. 5. A distance G controls the couplingdegree of the two SAW resonators. The smaller this distance is, thestronger the coupling degree becomes, which is preferable for attaininga broader band. If G becomes too small, however, widths W1 and W2 of thebus bar electrodes disposed in this part become much smaller so that theelectrical resistance loss of the IDT electrodes in this part affectsinsertion loss of the filter considerably. In this embodiment, a bus barelectrode 61 is directly connected to one of balanced input terminals,and a bus bar electrode 62 is used for grounding the IDT electrodecomprising the SAW resonator in the center, so they are not electricallyconnected to input-output terminals. In other words, W2 does not affectthe insertion loss of the filter at all. Therefore, when W2 is selectedto be smaller, W1 can be selected to be larger for compensation, andthen, the electrical resistance loss into the input-terminal can bereduced without changing the amount of G, thereby improving theinsertion loss of the filter. Furthermore, the same effect can beattained by constructing the part of an output side or the part whereIDT electrodes 54a and 53a are closely disposed to be of the samestructure.

Experiments were conducted under the conditions in which a filter of 240MHz was formed on the surface of a ST crystal substrate and a length ofG was one wavelength (about 12 μm). When both W1 and W2 had lengthes of0.25 wavelength, the insertion loss was 3.86 dB. On the other hand, whenW1 was 0.4 wavelength and W2 was 0.15 wavelength, the insertion loss was2.83 dB, which shows an improvement of 1.03 dB.

As shown in FIG. 7, this embodiment can be constructed in such a mannerthat a plurality of SAW filters 72 and 73 are concatenately connected onthe surface of the same piezoelectric substrate 71 to form a multistageconnected SAW filter. In this case, although the insertion lossincreases slightly, the out-of-band rejection characteristics improveconsiderably, so even better filter characteristics can be obtained.

If good pass characteristics are not obtained simply by connectingconcatenately, a reactive element such as an inductor should beconnected as a matching element to an electrode pattern 74 which servesfor an interstage concatenate connection. This method can be conductedby connecting the matching element between balanced electrodes 74, butthe same effect can be attained by connecting it between one ofelectrode patterns 74 and the earth and by grounding the other electrodeof 74. Here, connection with an external circuit is simplified when anelectrode pad 75 for bonding is disposed to electrode pattern 74. Or, byforming a reactive element such as a spiral inductor on the samepiezoelectric substrate and by connecting one end to electrode pattern74 while grounding the other end, an outside circuit is no longer neededso that a compact-sized filter circuit can be attained.

Accordingly, by forming the IDT electrodes of closely disposed SAWresonators to be electrically independent and by constructing it ofbalanced type input-output terminals, not only can the same effect as inthe first embodiment be obtained but also improved characteristics canbe obtained in this way.

EXAMPLE 3

FIG. 8 is a schematic view of a surface acoustic wave filter of a thirdembodiment of this invention. FIG. 8 shows that on the surface of amonocrystal piezoelectric substrate 81, there are a first SAW resonatorformed by an IDT electrode 82a and reflectors 82b, 82c and a third SAWresonator formed by an IDT electrode 84a and reflectors 84b, 84c. Theseelements form the same structure as of the second embodiment shown inFIG. 5. This figure differs from FIG. 5 in that an IDT electrode part ofa second SAW resonator formed in the center and accompanied byreflectors 83b, 83c has the same structure with the reflectors. Thisfigure also differs in the structure in which a periodic-structuredstripline electrode row 83a has about the same length with an electrodeoverlap width of IDT electrode 53a in FIG. 5.

As in the second embodiment, the above-noted three SAW resonators aredisposed close to each other, and the adjacent bus bar electrodes areelectrically independent. Furthermore, an electrode finger of IDTelectrode 82a is connected to a balanced type input terminal, while anelectrode finger of IDT electrode 84a is connected to a balanced typeoutput terminal. In addition, periodic-structured stripline electroderow 83a is grounded via an electrode pattern disposed in an aperture ofIDT electrode 82a and reflector 82b and also via an electrode patterndisposed in an aperture of IDT electrode 84a and reflector 84c.

According to the surface acoustic wave filter constructed in the abovemanner, the surface acoustic wave can propagate in the same way as longas the electrode cycle remains the same, even if the structure ofelectrode 83a changes from an IDT electrode to a periodic-structuredstripline electrode row in the central SAW resonator. Therefore, theoperation is basically the same as that of the first embodiment shown inFIG. 2, thereby achieving a broader band and suppressing the out-of-bandinterference of the SAW filter. In addition, as in the secondembodiment, since the bus bars in the central part of the IDT electrodesare electrically independent, a balanced input-output of the SAW filteris possible, and the out-of-band rejection characteristics of the filtercan be improved.

Next, FIG. 9 is an enlarged view of a part where IDT electrode 82a andperiodic-structured stripline electrode 83a are disposed close to eachother in FIG. 8. Comparing this figure with the second embodiment inFIG. 5, a proportion of a part in distance G where the electrodes arenot present reduces, and instead, the width W1 of a bus bar electrode 91can be enlarged in comparison with the width W2 of a bus bar electrode92. Accordingly, an insertion loss of the filter can be further improvedthan in the second embodiment.

According to the above-noted embodiment, not only can the same effect asin the second embodiment be obtained but also improved characteristicscan be obtained in this way.

Furthermore, IDT electrodes 82a and 84a in FIG. 8 are arranged to beequal in phase. Even if this structure is changed to be opposite inphase as with IDT electrodes 102, 104 formed on substrate 101 shown inFIG. 10, the same effect can be attained although the out-of-bandinterference appears slightly differently. According to FIG. 10, aperiodic-structured stripline electrode row 103 is grounded only via anaperture of IDT electrode 104 side. However, this kind of slightdifference in symmetry of the upper and lower electrode patternsscarcely affects the filter characteristics, and therefore, the wiringto the outside can be simplified.

Also in this embodiment, a plurality of SAW filters can be concatenatelyconnected to form a multistage connected SAW filter. In this way, theout-of-band rejection characteristics can be improved considerably,thereby obtaining even better filter characteristics. Here, the samemethod of concatenate connection as in the second embodiment shown inFIG. 7 can be applied.

Additionally in this embodiment, described in comparison with the secondembodiment, it was assumed that the structure of input-output electrodesof the SAW filter was balanced. However, the same effect can be attainedin comparison with the first embodiment with an unbalanced input-outputstructure as shown in FIG. 11 where 111 is a monocrystal piezoelectricsubstrate, 112, 114 are IDT electrodes, 113 is a periodic-structuredstripline electrode row, and common bus bars are formed at the partwhere the SAW resonators are disposed close to each other.

EXAMPLE 4

FIG. 12 is a schematic view of a surface acoustic wave filter of afourth embodiment of this invention. In FIG. 12, a first SAW resonatoris formed by IDT electrodes 122a, 122b and reflectors 123a, 124a, 123b,124b disposed on the surface of a monocrystal piezoelectric substrate121, and electrode fingers of IDT electrodes 122a, 122b are arranged tobe opposite in the phase relationship. On the same piezoelectricsubstrate 121, a second SAW resonator is formed by IDT electrodes 125a,125b and reflectors 126a, 127a, 126b, 127b, and electrode fingers of IDTelectrodes 125a, 125b are arranged to be equal in the phaserelationship. These two SAW filters are parallel-connected electricallyby the electrode patterns or circuit patterns 128a, 128b.

Next, operation of the surface acoustic wave filter constructed abovewill be explained.

FIGS. 13(a) and (b) show pass characteristics of a SAW filter of thisembodiment at direct-coupled 50Ω. In FIG. 13 (a), 131 and 132 are singlepass characteristics of either a first or a second SAW resonator filtershown in FIG. 12. In this way, a single SAW resonator filter has twoexcitation frequencies of f₁ and f₂ (f₁ <f₂) or f₃ and f₄ (f₃ <f₄), andthe phase relationship is opposite to each other. According to FIG. 12,the IDT electrodes in the first SAW resonator filter are arranged to beopposite in phase, while the IDT electrodes in the second SAW resonatorfilter are arranged to be equal in phase. Therefore, the phaserelationship between f₁ and f₂ becomes opposite to that between f₃ andf₄. In other words, f₂ and f₃ become equal in phase. As a result, byconforming the excitation frequencies of f₂ and f₃, the band width canbe broadened without deteriorating pass characteristics of the band, asshown in FIG. 13(b) 133.

It is actually difficult to conform f₂ with f₃ completely, so adifference appears more or less. When f₂ is smaller than f₃, thisdifference merely causes the pass band characteristics to deteriorategradually. When f₂ is larger than f₃, the phase changes greatly becauseof the overlapping peaks, and in particular, the group delay deviationcharacteristics are greatly disturbed. Thus, in view of the formationdeviation etc. of the SAW filter, the yield of the filter improves byshifting the peak value on the side of f₂ <f³.

Furthermore, by using an electrode pattern disposed on the surface ofthe piezoelectric substrate to form a part of the parallel-connected SAWfilters, stray capacitance of wire or undesired radiation etc. caused byinstalling bonding wire around can be prevented so that good filtercharacteristics can be obtained in this way.

According to the embodiment described above, the SAW filters whose IDTelectrode fingers are opposite in phase are parallel-connected, and oneof the high side excitation frequency was conformed to the other of thelow side excitation frequency. As a result, a band of the SAW multiplemode filter can be broadened without using an outside elongation coil orthe like.

In this embodiment, the electrode pattern was used only on one side toconduct the parallel connection, but naturally, electrode patterns canbe also used for all the connecting parts.

EXAMPLE 5

FIG. 14 is a schematic view of a surface acoustic wave filter of a fifthembodiment of this invention. In FIG. 14, SAW resonator filters 142,143, 144, 145 are formed on the surface of a piezoelectric substrate141. As with the first SAW resonator filter in FIG. 12, SAW resonatorfilters 142, 144 have adjacent electrode fingers arranged to be oppositein phase. Also as with the second SAW resonator filter in FIG. 12, SAWresonator filters 143, 145 have adjacent electrode fingers arranged tobe equal in phase. Then, SAW resonator filters 142, 143 and 144, 145 areparallel-connected respectively through bonding wire 146, 147 orelectrode patterns 148a, 148b, 148c. Furthermore, these two pairs ofparallel-connected SAW filters are concatenately connected throughelectrode patterns 148a and 148b. 149 is an absorber disposed forintercepting the propagation of surface acoustic waves caused by adifferent SAW resonator filter.

In the surface acoustic wave filter constructed in the above-notedmanner, the two pairs of parallel-connected SAW filters are respectivelyoperated as in the first embodiment. When these filters areconcatenately connected, the out-of-band rejection is improved evenmore.

Also in this embodiment, by using the electrode pattern disposed on thesurface of the piezoelectric substrate, the SAW filters areparallel-connected and concatenately connected. As a result, straycapacitance of wire or undesired radiation etc. caused by installingbonding wire around can be prevented so that good filter characteristicscan be obtained in this way.

In addition, by disposing the SAW filters having the samecharacteristics parallel and adjacent to the propagation direction ofthe surface acoustic wave, undesired stray capacitance is eliminatedbetween the concatenately connected multistage connected SAW filters,and therefore, an interstage matching circuit is no longer needed.Accordingly, the circuit can be compact-sized, while stablecharacteristics of the filter can be achieved.

Also in this case, if good pass characteristics can not be obtainedsimply by connecting concatenately, a matching element such as aninductor should be connected to electrode patterns 148a, 148b, and 148cwhich conduct the interstage concatenate connection.

As described above, by connecting parallel-connected SAW filterconcatenately, the same effect as in the fourth embodiment can beattained. At the same time, the characteristics can be improved evenmore.

In this embodiment, each stage is parallel-connected by an electrodepattern 148c. However, the same effect can be attained in a structurewithout the above-mentioned electrode pattern 148c by concatenatelyconnecting each SAW filter and then parallel-connecting each multistageconnected SAW filter.

EXAMPLE 6

FIG. 15 is a schematic view of a surface acoustic wave filter of a sixthembodiment of this invention. In FIG. 15, SAW resonator filters 152, 153are formed on the surface of a monocrystal piezoelectric substrate 151.152 have the adjacent IDT electrode fingers arranged to be opposite inphase, while 153 have the adjacent IDT fingers arranged to be equal inphase. These two SAW resonator filters are electricallyparallel-connected through electrode patterns 155a, 155b and throughwiring patterns 154a, 154b. This configuration is the same with that ofthe fourth embodiment shown in FIG. 12.

This embodiment differs from FIG. 12 in that a bus bar electrode 158 inthe central part of the adjacent IDT electrode fingers in SAW resonatorfilter 152 are divided to form electrically independent electrodes 156b,157b respectively. Also, SAW resonator filter 153 is constructed in sucha manner that the bus bar electrodes in the central part are divided.

FIG. 16 shows an example of the SAW filter of this embodiment which ismounted to a surface mounting package, and the same numerals are givento the parts corresponding to FIG. 15. 161 is a surface mounting packagewhere piezoelectric substrate 151 of the SAW filter is mounted. Then,162 is an electrode pattern for a bonding pad each connected by bondingwire 163 to input terminal electrodes 164a, 164b, output terminalelectrodes 165a, 165b, or to a grounded electrode 166 etc.

The surface acoustic wave filter constructed in such a manner isoperated basically in the same way as in the fourth embodiment shown inFIG. 13. Here, a band of the SAW filter can be broadened. In addition,by dividing the bus bars in the central part of the IDT electrodes,ground electrodes of an input IDT electrode part 154a, 154b and anoutput IDT electrode part 155a, 155b can be obtained independently. As aresult, the SAW filter can conduct balanced input-output by providingwiring shown as 154a, 154b, 155a, 155b.

According to this embodiment, the IDT electrodes of the SAW filter areno longer needed to be grounded. Thus, as in the second embodiment, theinput-output characteristics of the filter are not directly affected byhow the SAW electrodes are grounded. Furthermore, since directcomponents of signals between the input-output terminals decreaseconsiderably, the out-of-band rejection characteristics of the filtercan be improved. Also, a balanced type element such as an IC can beconnected to before and behind stages of the filter without using anoutside circuit of balun etc. As a result, noise characteristics of thewhole circuit are improved as well.

As shown in FIG. 16, by disposing the bonding pad on the surface of thepiezoelectric substrate and by connecting the terminal electrodes of thesurface mounting package to the bonding wire, a SAW filter that iscompatible with the surface mounting package can be attained.

In addition, this structure of dividing the bus bars in the central partof the SAW filter can be applied not only to the parallel-connected typeSAW filter shown in this embodiment, but also to the conventional SAWfilter shown in FIG. 24.

Also in this embodiment, the SAW filter was described assuming thatthere is one stage. Naturally, this method can be applied to themultistage SAW filter with two or more stages shown in the fifthembodiment of this invention.

EXAMPLE 7

FIG. 17 is a schematic view of a surface acoustic wave filter of aseventh embodiment of this invention. In FIG. 17, a first-stage SAWresonator filter comprising an IDT electrode 172 and reflectorelectrodes 173, 174 is formed on the surface of a piezoelectricsubstrate 171. On the surface of piezoelectric substrate 171, there isalso a second-stage SAW resonator filter comprising an IDT electrode 175and reflector electrodes 176, 177. The above-mentioned two stages of SAWresonator filters are concatenately connected electrically through aninterstage electrode pattern 178, thereby forming a multistage connectedSAW filter. Here, the first- and second-stage SAW resonator filters havetwo energy trapping type SAW resonators disposed close to each other toform an acoustic coupling.

Reflector electrodes 173 and 176 form meander line inductor electrodeswhich are each formed by bundling a plurarity of stripline electrodesand then connecting them in a zigzag pattern. The reflector electrodesare respectively parallel-connected between the electrodes on theinput-output side of IDT electrodes 172, 175 and the earth. Furthermore,reflector electrodes 174, 177 form inter-digital capacitor electrodeswhich are respectively serially connected to the electrodes on theinput-output side of IDT electrodes 172, 175.

Next, the operation of the surface acoustic wave filter constructedabove will be explained.

FIG. 18 is an equivalent circuit view of a SAW filter electrode patternin a seventh embodiment of this invention. The same numerals are givento the parts corresponding to that in FIG. 17. Here, 181 is afirst-stage SAW filter, and 182 is a second-stage SAW filter.

In FIG. 17, electrode patterns 173, 174, and 176, 177 are formed asacoustic reflectors of the SAW resonators comprising the SAW multiplemode filter. When each electrode is seen as a transmission line, 173,176 function as meander line inductors and 174, 177 as inter-digitalcapacitors. Therefore, these reflector electrodes are connected to theinput-output side electrodes of IDT electrodes 172, 175 as describedabove to form a matching circuit shown in FIG. 18.

Accordingly, a reactive electrode pattern disposed on the surface ofpiezoelectric substrate 171 functions as the matching circuit of the SAWfilter. As a result, an exterior matching circuit is no longer needed sothat a number of components can be reduced, thereby miniatuarizing thewhole circuit. Furthermore, since the above-noted reactive electrodepattern is formed by using the reflector electrodes of the SAWresonator, the circuit structure above can be attained with about thesame piezoelectric substrate area as with a conventional filter.

Furthermore, when the reactive elements are formed by the striplineelectrodes of the reflectors, differences in the element amount can bereduced in comparison with the case using exterior circuit elements, sothe circuit characteristics of the SAW filter can be even morestablized. Also, when the meander lines are constructed such that theycomprise several reflector stripline electrodes per one direction (twolines each in 173, 176 of FIG. 17), the resistance component of theinductor can be reduced to prevent the filter characteristics fromdeteriorating. The number of reflectors per one line should be selectedsuitably according to the necessary inductance value and resistancevalue.

As described above, this embodiment can attain a SAW filter in which thefilter matching circuits are combined together on the surface of thepiezoelectric substrate.

In this embodiment, the matching circuits were described as an examplein which the inductors were parallel-connected and the capacitors wereserially connected. However, the structure is not limited to this type,and the circuit structure can be changed flexibly according to theimpedance of the SAW filter.

EXAMPLE 8

FIG. 19 is a schematic view of a surface acoustic wave filter of aeighth embodiment of this invention. In FIG. 19, a first-stage SAWresonator filter comprising an IDT electrode 192 and reflectorelectrodes 193, 194 is formed on the surface of a piezoelectricsubstrate 191. A second-stage SAW resonator filter comprising an IDTelectrode 195 and reflector electrodes 196, 197 is formed onpiezoelectric substrate 191. The above-mentioned two stages of SAWresonator filters are concatenately connected electrically through aninterstage electrode pattern 198 to form a multistage connected SAWfilter. Here, the first- and the second-stage SAW resonator filters areconstructed such that the two energy trapping type SAW resonators aredisposed close to each other to form an acoustic coupling.

Reflector electrodes 193 and 196 form meander line inductor electrodeswhich are each formed by bundling a plurarity of stripline electrodesand then connecting them in a zigzag pattern. Then, a short-circuitelectrode is formed in the center of the stripline electrode ofreflector electrode 193, and in reflector electrode 196, the meanderline is folded back in the center of the stripline electrode.Furthermore, reflector electrodes 194, 197 comprise inter-digitalcapacitor electrodes. The input-output terminals are taken out from thecommon bus bar parts in the center of reflector electrodes 194, 197which are each formed such that the electrode inconsecutive parts in thecentral part of the reflectors are excluded as much as possible.

In the SAW filter constructed above, as in the seventh embodiment, thematching circuits at the input-output stages are formed on the surfaceof the same piezoelectric substrate 191, and at the same time, thecommon bus bar electrodes of the reflector parts are present. As aresult, undesired coupling between the closely disposed SAW resonatorsis suppressed, thereby obtaining even better filter characteristics.

Besides in this embodiment, the circuit structure can be changedflexibly according to impedance of the SAW filter.

EXAMPLE 9

FIG. 20 is a schematic view of a surface acoustic wave filter of a ninthembodiment of this invention. In FIG. 20, a first-stage SAW resonatorfilter comprising an IDT electrode 202 and reflector electrodes 203, 204is formed on the surface of a piezoelectric substrate 201. Also, asecond-stage SAW resonator filter comprising an IDT electrode 205 andreflector electrodes 206, 207 is formed on the surface of piezoelectricsubstrate 201. The above-mentioned two stages of SAW resonator filtersare concatenately connected electrically through an interstage electrodepattern 208 to form a multistage connected SAW filter. Here, the first-and the second-stage SAW resonator filters are constructed such that thetwo energy trapping type SAW resonators are disposed close to each otherto form an acoustic coupling.

Reflector electrodes 203, 206 form meander line inductor electrodeswhich are formed by connecting stripline electrodes in a zigzag patternwhich are parallel-connected respectively between an interstageelectrode pattern 208 of IDT electrodes 202, 205 and the earth.Furthermore, reflector electrodes 204, 207 comprise inter-digitalcapacitor electrodes which are serially connected respectively to theinput-output side electrodes of IDT electrodes 202, 205.

Next, the operation of the surface acoustic wave filter constructedabove will be explained.

FIG. 21 is an equivalent circuit view of a surface acoustic wave filterelectrode pattern in a ninth embodiment of this invention. The samenumerals are given to the parts corresponding to FIG. 20. Here, 211shows a first-stage SAW resonator filter, and 212 is a second-stage SAWresonator filter. As shown in FIG. 12, it is clear that reflectorelectrodes 204 and 207 function as a part of input-output matchingcircuits as in the seventh embodiment, however, reflector electrodes 203and 206 function as matching elements for the filter interstage.

By connecting inductor elements comprising reflector electrodes at thefilter interstage, an interstage unadjustment of a SAW filter can beachieved without newly increasing an electrode area on the surface ofthe piezoelectric substrate. It is no longer necessary to connect anadjustment circuit such as an elongation coil at the interstage of,e.g., a broadband type transversely coupled resonator type SAW filter.

Furthermore, the inductor elements of reflector electrodes 203 and 206are parallel-connected respectively to the interstage electrode patternsso that an inductance value can be adjusted by cutting off one of theelectrode patterns.

According to the above-mentioned embodiment, a SAW filter can beobtained in which the interstage adjustment circuits of the filter arealso integrated on the surface of the piezoelectric substrate.

In this embodiment, the inductors were parallel-connected at the filterinterstage, and the matching circuits were arranged such that thecapacitors were serially connected at the input-output stages. However,the structure is not limited to this type, and the circuit structure canbe changed optionally according to impedance of the SAW filter. Inaddition, if matching circuit elements comprising the reflectorelectrodes are lacking, a reactive electrode pattern can be newly formedon the surface of the piezoelectric substrate, or it can be connected toan outside circuit.

Furthermore, the reflector electrode patterns in this embodiment can bereplaced with the reflector electrode patterns having common bus barsshown in the eighth embodiment.

EXAMPLE 10

FIG. 22 is a schematic view of a surface acoustic wave filter of a tenthembodiment of this invention. In FIG. 22, a first-stage SAW resonatorfilter comprising an IDT electrode 222 and reflector electrodes 223, 224is formed on the surface of a piezoelectric substrate 221. Asecond-stage SAW resonator filter comprising an IDT electrode 225 andreflector electrodes 226, 227 is also formed on the surface ofpiezoelectric substrate 221. The above-mentioned two stages of SAWresonator filters are concatenately connected electrically through aninterstage electrode pattern 228 to form a multistage connected SAWfilter. Here, the first- and the second-stage SAW resonator filters havetwo energy trapping type SAW resonators disposed close to each otherwhich form an acoustic coupling.

Reflector electrodes 223, 226 form meander line inductor electrodeswhich are formed by bundling and connecting a plurality of striplineelectrodes in a zigzag. They are parallel-connected respectively betweeninput-output electrodes of IDT electrodes 222, 225 and the earth.Furthermore, reflector electrodes 224, 227 comprise inter-digitalcapacitor electrodes which are serially connected respectively to theinput-output electrodes of IDT electrodes 222, 225. Then, theabove-mentioned interstage electrode pattern 228 is grounded onpiezoelectric substrate 221 via a newly formed spiral inductor electrodepattern 229.

According to the thus formed surface acoustic wave filter, circuits of aserial capacitor and a parallel capacitor are formed at input-outputstages, as in the equivalent circuit shown in FIG. 18. Furthermore, thecircuit comprises a parallel inductor at the interstage of the SAWfilter. Therefore, as in the seventh embodiment, matching circuits atthe input-output stages can be formed on the surface of the samepiezoelectric substrate 221, and at the same time, an interstageadjustment circuit described in the ninth embodiment can be formed onthe surface of the same piezoelectric substrate.

In addition, the first-stage SAW resonator filter and the second-stageSAW resonator filter must be formed apart from each other to some degreeto prevent them from forming undesired acoustic coupling. Thus, as shownin FIG. 22, the above-noted spiral inductor electrode pattern 229 can beformed by using an space between the SAW resonator filters, so it isunnecessary to enlarge the piezoelectric substrate more than in aconventional filter. The circuit can be miniatuarized in this way.

According to the embodiment mentioned above, the same effect as in theseventh embodiment can be attained. Also, this embodiment is suitablewhere an interstage adjustment circuit is needed for the filter as shownin FIG. 9.

In this embodiment, the matching circuit was explained such that theinductor was parallel-connected between the filter stages, and that theinductors were parallel-connected at the input-output stages, while thecapacitors were serially connected. However, the structure is notlimited to this type, and the circuit structure can be changedoptionally according to the impedance of the SAW filter. In addition, itis also possible to form only a part of this reactive element for usethereof.

Furthermore, the reflector electrode patterns in this embodiment can bereplaced with the reflector electrode patterns having common bus bar inthe eighth embodiment.

Additionally, as shown in FIG. 23(a), when a short-circuit electrodepattern 231 is formed to a spiral inductor electrode pattern 229 for ashort-circuit connection between the adjacent winding electrodepatterns, the reactive value can be finely controlled by trimming theabove-noted short-circuit electrode pattern by means of a laser or thelike. As a result, the filter characteristics can be finely controlledafter mounting the SAW filter substrate to the package.

Furthermore, in the seventh, ninth, and tenth embodiments, each reactivevalue can be finely controlled by trimming the part of electrode pattern232 in FIG. 23(b) in case of meander line inductor electrodes 173, 203,223 etc. and also by trimming the part of electrode pattern 233 in FIG.23(c) in case of inter-digital capacitor electrodes 174, 204, 224 etc.

Also in the seventh, ninth, and tenth embodiments, the descriptionreferred only to an example of a two-stage concatenately connected SAWfilter. It is clear that this invention includes a general resonatortype SAW filter or a multielectrode type SAW filter and can be appliedto the entire surface acoustic wave elements accompanied by reflectors.

Furthermore, it is preferable that ST cut crystal with excellent thermalcharacteristics is used for the piezoelectric substrate of thisinvention. However, it is also possible to use substrates such asLiTaO₃, LiNbO₃, Li₂ B₄ O₇. In addition, it is preferable that aluminiumwith comparatively low density is used for the electrode material fromthe viewpoint that the electrode thickness can be controlled easily, butgold electrodes are also applicable. Also, it is clear that thisinvention can be applied not only to resonators using a surface acousticwave of Rayleigh wave, but also to resonators using a surface skimmingbulk wave (SSBW) or a pseudo surface acoustic wave (Leaky SAW) etc.

The invention may be embodied in other forms without departing from thespirit or essential characteristics thereof. The embodiments disclosedin this application are to be considered in all respects as illustrativeand not restrictive, the scope of the invention is indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. A surface acoustic wave (SAW) filter comprising aSAW resonator disposed on a surface of a piezoelectric substrate, theSAW resonator comprising at least three pieces, an inter-digitaltransducer (IDT) electrode having at least four sides disposed in thecenter of the SAW filter and reflectors disposed adjacent to two sidesof the IDT electrode, wherein said SAW resonator form an acousticcoupling by disposing the at least three pieces close to each other andin parallel to a propagation direction of the SAW resonator, wherein theIDT electrode positioned in the center is totally grounded on at leastthe two sides of the IDT electrode not adjacent to the reflectors, andwherein other IDT electrodes of said SAW filter disposed outside saidcenter are electrically independent.
 2. The surface acoustic wave filteras in claim 1, wherein said IDT electrode comprising the SAW resonatorpositioned in the center is grounded via electrode patterns disposedbetween the IDT electrodes of the SAW resonators disposed outside saidcenter and electrodes of the reflectors.
 3. The surface acoustic wavefilter as in claim 1, wherein a plurality of said filter isconcatenately connected on the surface of the piezoelectric substratethrough an interstage connecting electrode pattern formed thereon. 4.The surface acoustic wave filter as in claim 3, wherein a part of saidinterstage connecting electrode pattern has an electrode pad formed forbonding.
 5. The surface acoustic wave filter as in claim 3, wherein saidinterstage connecting electrode pattern is grounded via a reactiveelement formed by an electrode pattern on the surface of thepiezoelectric substrate.
 6. The surface acoustic wave filter as in claim5, wherein said reactive element is a spiral inductor.
 7. A surfaceacoustic wave (SAW) filter, comprising two SAW resonators, each of saidtwo SAW resonators comprising an inter-digital transducer (IDT)electrode having at least four sides and reflectors adjacent to twosides of the IDT electrode, the two SAW resonators being disposed on asurface of a piezoelectric substrate, wherein said SAW resonators areformed in parallel to a propagation direction of said SAW resonators,the IDT electrodes are grounded on at least the two sides of the IDTelectrodes not adjacent to the reflectors, and between said SAWresonators, a periodic-structured electrode row is present comprisingstripline electrodes having about the same length as an IDT electrodeoverlap width of said SAW resonators, wherein said stripline electrodesare parallel-disposed at the same electrode period as in said SAWresonators, and said SAW resonators and said periodic-structuredelectrode row form an acoustic coupling by being disposed close to eachother.
 8. The surface acoustic wave filter as in claim 7, wherein eachstripline electrode comprising the periodic-structured electrode row isconnected to each other through bus bars disposed on both edges.
 9. Thesurface acoustic wave filter as in claim 8, wherein saidperiodic-structured electrode row is grounded via electrodes disposed inan aperture between the IDT electrodes of the SAW resonators positionedin the outside and electrodes of the reflectors and via bus barelectrodes.
 10. The surface acoustic wave filter as in claim 9, whereinan electrode of one SAW resonator positioned in the outside is connectedto a balanced type input terminal, and an electrode of the other SAWresonator positioned in the outside is connected to a balanced typeoutput terminal.
 11. The surface acoustic wave filter as in claim 10,wherein a line width of the electrode patterns for bus bars on adjacentside of said periodic-structured electrode row of the IDT electrodescomprising the SAW resonators positioned in the outside is selected tobe larger than a line width of the electrode patterns for bus barsformed on said periodic-structured electrode row.
 12. The surfaceacoustic wave filter as in claim 7, wherein a plurality of said filteris concatenately connected through several interstage connectingelectrode patterns formed on the surface of the piezoelectric substrate.13. The surface acoustic wave filter as in claim 8, wherein IDTelectrodes on the adjacent side of the periodic-structured electrode rowof the SAW resonator are integrated with bus bar electrodes whichconnect said periodic-structured electrode row, and saidperiodic-structured electrode row is grounded.
 14. The surface acousticwave filter as in claim 13, wherein a plurality of said filter isconcatenately connected through several interstage connecting electrodepatterns formed on the surface of the piezoelectric substrate.
 15. Asurface acoustic wave (SAW) filter comprising two SAW resonatorsdisposed on a surface of a piezoelectric substrate, a first SAWresonator comprising at least three pieces, an inter-digital transducer(IDT) electrode having at least four sides and two reflectors whereinthe IDT electrode is disposed between the two reflectors, wherein saidIDT electrode is grounded on at least two sides of the IDT electrode notadjacent to the reflectors, wherein the two SAW resonators form anacoustic coupling by being disposed close to each other, whereinelectrodes of the first SAW resonator are arranged to be opposite inphase, and electrodes of a second SAW resonator are arranged to be equalin phase, and wherein said first SAW resonator and said second SAWresonator are parallel-connected.
 16. The surface acoustic wave filteras in claim 15, wherein the first and the second SAW resonator filtersare arranged so that a high band side excitation frequency of one SAWresonator filter conforms with a low band side excitation frequency ofthe other SAW resonator filter.
 17. A surface acoustic wave (SAW) filtercomprising a SAW resonator disposed on a surface of a piezoelectricsubstrate, the SAW resonator comprising at least five pieces, aninter-digital transducer (IDT) electrode having at least four sides andreflectors disposed adjacent to two sides of the IDT electrode, whereinsaid IDT electrode is grounded on at least the two sides of the IDTelectrode not adjacent to the reflectors, wherein said SAW resonatorform an acoustic coupling by disposing the at least five pieces close toeach other, wherein electrodes of the SAW resonator comprising a firstand a second SAW resonator filter are arranged to be opposite in phase,and electrodes of the SAW resonator comprising a third and a fourth SAWresonator filter are arranged to be equal in phase, wherein said firstSAW resonator filter and said third SAW resonator filter areparallel-connected and said second and said fourth SAW resonator filtersare parallel-connected, and wherein said first and said third SAWresonator filters and said second and said fourth SAW resonator filtersare concatenately connected through electrode patterns formed betweensaid filters on the surface of said piezoelectric substrate.
 18. Thesurface acoustic wave filter as in claim 17, wherein the first and thethird SAW resonator filters are arranged so that a high band sideexcitation frequency of one SAW resonator filter conforms with a lowband side excitation frequency of the other SAW resonator filter, andthe second and the fourth SAW resonator filters are arranged so that ahigh band side excitation frequency of one SAW resonator filter conformswith a low band side excitation frequency.
 19. The surface acoustic wavefilter as in claim 17, wherein the first SAW resonator filter and thesecond SAW resonator filter are positioned next to each other inparallel to a propagation direction of the surface acoustic wave, andthe third SAW resonator filter and the fourth SAW resonator filter arepositioned next to each other in parallel to a propagation direction ofthe surface acoustic wave.
 20. A surface acoustic wave (SAW) filtercomprising a SAW resonator disposed on a surface of a piezoelectricsubstrate, the SAW resonator comprising an inter-digital transducer(IDT) electrode having at least four sides and reflectors adjacent totwo sides of the IDT electrode, wherein the IDT electrode is grounded onat least the two sides of the IDT electrode not adjacent to thereflectors, wherein the SAW resonator forms acoustic coupling bydisposing the IDT electrode and reflectors close to each other, whereinelectrode patterns for bus bars are divided at a central part of anelectrode adjacent to the SAW resonator, and wherein other SAWresonators disposed on the piezoelectric substrate have electricallyindependent bus bars.
 21. The surface acoustic wave filter as in claim20, wherein two pieces of SAW filter are formed on the samepiezoelectric substrate, and an electrode of the SAW resonatorcomprising a first SAW resonator filter is arranged to be opposite inphase, and an electrode of the SAW resonator comprising a second SAWresonator filter is arranged to be equal in phase, wherein said firstSAW resonator filter and said second SAW resonator filter areparallel-connected.
 22. The surface acoustic wave filter as in claim 21,wherein the first and the second SAW resonator filters are constructedin such manner that a high band side excitation frequency of one SAWresonator filter conforms with a low band side excitation frequency ofthe other SAW resonator filter.
 23. A surface acoustic wave (SAW) filtercomprising a SAW resonator disposed on a surface of a piezoelectricsubstrate, the SAW resonator comprising at least five pieces, aninter-digital transducer (IDT) electrode having at least four sidesdisposed in the center of the SAW filter and reflectors disposedadjacent to two sides of the IDT electrode, wherein said IDT electrodeis grounded on at least the two sides of the IDT electrode not adjacentto the reflectors, wherein the SAW resonator forms an acoustic couplingby disposing the at least five pieces close to each other, whereinelectrode patterns for bus bars are divided at the central part of theSAW resonator, and the at least five pieces of said SAW resonator arepresent such that electrodes of the SAW resonator comprising a first anda second SAW resonator filter are arranged to be opposite in phase, andelectrodes of the SAW resonators comprising a third and a fourth SAWresonator filter are arranged to be equal in phase, and said first SAWresonator filter and said third SAW resonator filter areparallel-connected and said second and said fourth SAW resonator filtersare parallel-connected, and wherein said first and said third SAWresonator filters are concatenately connected to said second and saidfourth SAW resonator filters through electrode patterns formed betweenthe filters on the surface of said piezoelectric substrate.
 24. Thesurface acoustic wave filter as in claim 23, wherein the first and thethird SAW resonator filters are arranged so that a high band sideexcitation frequency of one SAW resonator filter conforms with a lowband side excitation frequency of the other SAW resonator filter, andthe second and the fourth SAW resonator filters are arranged so that ahigh band side excitation frequency of one SAW resonator filter conformswith a low band side excitation frequency of the other SAW resonatorfilter.
 25. A surface acoustic wave (SAW) filter comprising a reactiveelement formed by using a part of electrode patterns of a SAW filter,wherein the reactive element is disposed with an inter-digitaltransducer (IDT) electrode having at least four sides and reflectorelectrodes, said reactive element is formed adjacent to said reflectorelectrodes, and wherein said IDT electrode is grounded on at least twosides of the IDT electrode not adjacent to the reflector electrodes. 26.The surface acoustic wave filter as in claim 25, wherein said reactiveelement is an inductor formed by connecting parallel-positionedstripline electrodes comprising the reflector electrodes in a zigzagpattern.
 27. The surface acoustic wave filter as in claim 25, whereinsaid reactive element is an inductor formed by bundling and connecting aplurality of parallel-positioned stripline electrodes comprising thereflector electrodes in a zigzag pattern.
 28. The surface acoustic wavefilter as in claim 25, wherein said reactive element is a capacitorformed by connecting parallel-positioned stripline electrodes comprisingthe reflector electrodes in an inter-digital form.
 29. The surfaceacoustic wave filter as in claim 25, wherein said reactive element formsan input-output matching circuit.
 30. The surface acoustic wave filteras in claim 25, wherein said reactive element forms an interstagematching circuit.
 31. The surface acoustic wave filter as in claim 25,wherein a plurality of SAW resonator comprising an IDT electrode andreflectors on both sides forms an acoustic coupling by being disposedclose to each other.
 32. The surface acoustic wave filter as in claim31, wherein said reactive element is formed with a reflector electrode.33. A surface acoustic wave (SAW) filter comprising a SAW resonatordisposed with an inter-digital transducer (IDT) electrode having atleast four sides and reflectors disposed adjacent to two sides of theIDT electrode, wherein said IDT electrode is grounded on at least thetwo sides of the IDT electrode not adjacent to the reflectors, wherein aplurality of SAW resonators form an acoustic coupling by being disposedclose to each other on a surface of a same piezoelectric substrate,wherein said plurality of SAW resonators are concatenately connected,and wherein an input-output matching circuit is formed by using areactive element formed by an electrode pattern disposed on the surfaceof said piezoelectric substrate.
 34. The surface acoustic wave filter asin claim 33, wherein said reactive element is formed by using reflectorelectrodes.
 35. A surface acoustic wave (SAW) filter comprising a SAWresonator disposed on a surface of a piezoelectric substrate, the SAWresonator comprising an inter-digital transducer (IDT) electrode in thecenter of the SAW filter, said IDT electrode having at least four sides,and reflectors adjacent to two sides of the IDT electrode, wherein saidIDT electrode is grounded on at least the two sides of the IDT electrodenot adjacent to the reflectors, wherein a plurality of SAW resonatorsform an acoustic coupling by being disposed closely to each other on thesurface of the piezoelectric substrate, wherein said plurality of SAWresonators are concatenately connected, and wherein connecting points ofsaid plurality of SAW resonators are grounded via a reactive elementformed by an electrode pattern disposed on the surface of thepiezoelectric substrate.
 36. The surface acoustic wave filter as inclaim 35, wherein said reactive element is a spiral inductor.
 37. Thesurface acoustic wave filter as in claim 36, wherein said spiralinductor is formed by using an aperture between the plurality of SAWfilters.
 38. The surface acoustic wave filter as in claim 36, wherein ashort-circuit electrode pattern for a short-circuit connection betweenthe winding electrode patterns adjacent to the sprial inductor isdisposed at least at one place.